Small crafts, such as fishing boats, fishing leisure boats, and the like, are frequently required to travel at quite low-speeds for some uses. For example, the following cases can be cited: stopping at fishing grounds; crafts stopping at a fixed point against a current without anchoring; traveling at a slow speed in accordance with the net hauling speed so as not to apply an excessive load to the net and to avoid tangling the net around the propeller during net hauling, etc.
As a device for achieving such travel, a trolling device mounted on a speed reducing and reversing apparatus provided with a friction disc hydraulic clutch is conventionally known (Patent Document 1).
The trolling device is so constituted that the speed of the craft can be adjusted by alternatively selecting an oil pressure at which a clutch can be fully connected and an oil pressure at which the clutch can be incompletely connected (half-clutch state) by rotating an operation lever.
With increase in engine outputs in marine gears, higher capacity of transmission torque is required. Increasing the number of friction discs in the clutches or the like can increase the capacity of the transmission torque, but doing so may cause problems such as an increased size of the marine gear body.
Accordingly, in order to increase the capacity of the transmission torque and also maintain the size of the marine gear body, the surface pressure of the friction discs needs to be raised to increase of the oil pressure setting of the marine gear.
[Patent Document 1] Japanese Unexamined Patent Publication No. H06-80098